I know just the feeling. I got busy at work one winter and didn't ride or work out at all. In the spring it was all I could do to average 16. Disgusting. Just ride your bike and keep a positive attitude. Remember that nothing happens if you don't go out. Every ride you'll get better. But you won't get the reinforcement without the riding! You'll be shocked at how fast it comes back. Just build the mileage and don't worry about it.